Wave-transferring system



Patented Dec. 3, 1929 UNITED STATES PATENT OFFICE NEW YORK RALPH V. L.HARTLEY, OF SOUTH ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRICCOMPANY, INCORPORATED, OF NET/V YORK, N. Y.,

A CORIORATION OF VJAVE-TRANSFERRING SYSTEM Application fi1ed December29, 1924, Serial No. 758,531. Renewed January 10, 1929.

.This'invention relates to wave transmis' sion, especially incommunication systems, such for instance as telephonesystems subect tolnterference' or disturbances, for example, interference of the typefrequently exthe disturbances, and, if desired, to also im-' prove theform of the received signals, for instance by compensating for anon-uniform line attenuation.

The effect of any external disturbances can be made negligible ifsufficient energy is used at the transmitting end of the system and'jsuflicient uniform or distortionless attenuation at the receiving end,but the permissible amplification at the sending end is limited by therestrictions which are imposed on the sending energy, since theapparatus, such for instance as the amplifying means, at that end mustnot be unduly overloaded. In accordance with the invention, the effectof a given amount of interference or disturbance is minimized with agiven sending energy, or the sendin ener necessar to secure a iven by beffected in the energy spectrum of the signal (that is, in the frequencydistribution of the signal energy) and the noise energy which arefinally produced at the receiver; This may be accomplished, forinstance, by the introduction of distortion at the receiving end and acompensating distortion at the sending end, the sending end distortionspreferably being complementary to the receiving end distortion (and anyline distortion), so that there will be no resultant or overalldistortion of the signals transmitted. The com plementary distortionsare effected by networks, one at the receiving end of the system and oneat the sending end; and an amplifier at the sending end preferablyamplifies the signals delivered from the sending end network tocompensate for a partor all of the attenuation produced by the twonetworks. The attenuation frequency characteristics or" the networksshould be such that the reduction of apparent loudness ofthe noisecaused by the receiving end network will exceed the difference betweenthe combined attenuations (of signals) introduced by the networks andthe decrease in equivalent load on the sending end of the system whichresults from passing undistorted signals through the sending network.The equivalent load referred to here is the energy level of theundistorted signals of which the sending energy is an equivalent asregards the load limit of the apparatus carrying the sending energy.

The frequency spectrum of signal energy usually (as for instance is thecase with the frequency spectrum of the energy of normal speech)contains frequencies of comparative-V 1y high energy level andfrequencies of comparatively low energy level. By means of thisinvention, not only can the effect of noise currents be reduced withoutincreasing the total power at the sending end of the system, but thepart of the noise represented by j currents of the latter frequenciescan be reduced or substantially eliminated by attenuating orsubstantially extinguishing those noise currents in the receiving endnetwork. This can be done without necessitating unduly increasing thelikelihood of overloading the amplifier or other apparatus at thesending end, even the load capacity of such amplifier or such otherapparatus be more dependent upon peak values, or single frequency, highvalues of energy than upon the energy integral over the whole frequencyspectrum of the currents transmitted; for the complementary network atthe sending end can be given high attenuating power at the frequenciesat which the signal energy is high, and will therefore prevent the gainintroduced by the amplifier to overcome the signal energy losses of thetwo networks from increasing the energy levels at the last mentionedfrequencies to values materially higher than obtained before theinsertion of the networks and before the increase of the amplifier gain.For example, in the hypothetical case in which the frequency spectrum ofthe noise energy incoming into the receiving end of a system iscomplementary to that of the signal energy, it would be possible toemploy a receiving end network having at every frequency an attenuatingpower just sufficient to substantially extinguish the ultimatelyreceived noise current of that frequency (an infinite attenuating powerwould of course be necessary were total extinction required), withoutnecessitating any substantial alteration of the highest energy leveldelivered at any frequency at the output of the amplifier at the sendingend; for there could be employed at the sending end a complementarynetwork having at each frequency an attenuating power just sufficient toalmost extinguish the signal current fed to the amplifier at thatfrequency, so that upon increase of the gain of the amplifier by anamount equal to the maximum attenuation of either network (the maximawould be the same for networks complementary to each other) ,the energydelivered at each frequency at the output of the amplifier would have alevel not exceeding the maximum level which obtained before theinsertion of the networks and the increase of the amplifier gain.

Although the invention is set forth hereinafter with especial referenceto a telephone system in which thedisturbing effects of noise currentsupon signals in the speech frequency range are reduced, the broadfeatures of the invention are of general application inwavetransmission, as, for example,-in the reduction of the effects ofinterference upon transmission in carrier and radio frequency ranges.

In the accompanying drawings, Fig. 1 is a diagram of a radio telephonesystem embodying one form of the invention; and Figs. 2 to 5 are curvesfor facilitating explanation of the invention.

In Fig. 1 two-way transmission, for instance two-way telephonictransmission, is carried on between signaling stations 10 and 10,through lines 11 and 11 and a wire and radio link between those lines,the link comprising a line 12, radio transmitter 13, radio receiver 1 1,and line for transmitting from line 11 to line 11, and the link furthercomprising a line 12, radio transmitter 13, radio receiver 1 1 and line15 7 for transmitting from line 11 to line 11. The connection of line 11to lines 12 and 15,and the connection of line 11 to lines 12 and 15 isaccomplished by means of three-winding transformers or hybrid coils 16and 16 and line balancing networks 17 and 17 as usual. The carrier wavegenerated at the radio transmitter 13 and received through the radioreceiver 14 is preferably of a frequency different from that of thecarrier wave generated at the radio transmitter 13' and received throughthe radio receiver 1 1. As is indicated by the clashes in lines 11, 12,15, 11', 12 and 15, these lines may be of any length.

In a system such as that of Fig. 1, considerable interference is likelyto be experienced, due especially to the reception, by the radioreceiver 1 1 of energy other thanthat radiated from the transmitter 13,and to the reception by the radio receiver 1 1 of energy other than thatradiated from the transmitter 13. To reduce the effects of suchinterference without necessitating increase of the sending energytransmitted into line 12 or 12, distorting networks 20, 21, 20" and 21,and amplifiers 30 and 30, are provided The attenuating network 20 isatt-he sending end of the channel for transmitting fromline 11 to line11', which for convenience may be termedthe east channel; theattenuating net work 21 is at the receiving end of that channel; theattenuating network 20 is at the sending end of the channel fortransmitting from line 11 to line 11, which may be called the westchannel; and the attenuatingnetwork 21 is at the receiving end of thatchannel. In explaining the operation of the system to reduceinterference effects it will be assumed that frequencies "outside of apredetermined desired frequency range are so highly attenuated beforereaching amplifier 80 or 30 that they may be neglected; and that, withinthe limits of their load capacity, the amplifiers transmit waves ef thepre'determined desired frequency range without distortion. Theseconditions can easily be approximated with sufficient closeness, inpractice. V h i It will now be explained how, in trans mitting from line11 to line 11, by having the network 21 at the receiving end of thechannel east introduce distortion of a suitable character, and havingthe network 20 intro: duce a compensating idlStOIliiOIl at the sendingend, the disturbing effect of a given amount of interference can bereduced without necessitating increase of sending energy. The manner inwhich the channel wes topera'tes to reduce the disturbing effect ofinterference when the system is transmitting from line 11 to line 11 issimilar, and will then be apparent without further explanation.

Consider first the case where the portion of the channel east betweenamplifier '30 and network 21 is free from distortion. It will be assumedthat, for speech frequencies in the predetermined desired frequencyrange menitoned above and extending from frequency f to frequency f inFigs. 2 to 5, the apparatus to the west of network 20 is free fromdistortion, a condition which can easily be pr imat ,s fieent ydQSdy pralit tice. It will also be'assumed that the magnitude of the receivedsound is not varied over so wide a range but what the reduction inintelligibility due to noise is unaffected by uniformly attenuating oramplifying both speech and noise by the same amount. As a method ofexpressing the magnitude and the frequency distribution of speech andnoise energy, the energy (that is, the energy per unit of time, or thepower associated with any frequency) will be expressed on a log arithmicscale the zero of which is the energy of that same frequency inundistorted speech which has been attenuated so as to be just at thethreshold of audition. The absolute value of this zero is immaterial forpresent purposes. Consider the conditions in the channel east before theintroduction of its distorting networks 20 and 21. Since the speecharriving at the east end of the channel after transmission through thechannel is assumed undistorted in the frequency range from f to f andbelow audibility outside that range, it may be represented by thehorizontal line a in Fig. 2. The distribution of the noise arriving atthe east end of the channel being in general different from that ofspeech let it be represented by the curved line b. This is placed belowa to indicate that the noise is smaller in magnitude than the speech.

Now it has been found to be at least approximately true that the effectof any disturbing noise on the intelligibility of speech is the same asthat of a sound having the energy distribution of undistorted speech andthe same apparent loudness as the noise. Let the curve 0 of Fig. 2represent the mag nitude of undistorted speech which is of the sameapparent loudness as the noise represented by curve I). (This magnitude,or in other words the height of curve 0, can be calculated by empiricalmethods indicated in the article by Fletcher and Steinberg in thePhysical Review, September, 1924, vol. 24, second series, No. 3, page306.) The distance between a and 0 or a0, expressed in transmissionunits is a measure of the dis turbing effect. (The transmission unithere referred to may be, for instance, that discussed in my article onThe transmission unit, in Electrical Communication, July,-

1924, published by the International WVestern Electric Company,Incorporated, New York. This unit is also discussed in the followingarticles: phone transmission systems, by W. H. Martin, and Practicalapplication of the transmission unit, by C. W. Smith, both in TheBellSystem Technical Journal, July, 1924. published by the AmericanTelephone and Telegraph Company, New York.)

Now let there be inserted at the east or receiving end of the channelthe network 21, with loss vs. frequency characteristic such, say, as isgiven by curve (Z, Fig. 3. The re- The transmission unit and teleeceived speech and noise are then roughly as shown by a 6 and 0 of Fig.4, where'the curve 0 is again the undistorted speech of the sameloudness as the noise. Curves a and c of Fig. 1 are reproduced in dottedlines;

In order to restore the speech let there be inserted at the sending orwest end of the channel the network 20 having the loss characteristic 6,Fig. 3, and introducing distortion complementary to that of network 21.This has no effect on the noise, but changes the received speech from (Lto ta If the speech energy (or power) at the sending end of the channelwas originally 9, Fig. 5, it is changed to 9 by the introduction of thesending end or west network. Since we have decreased the sending energy,it will now be possible to introduce, by amplifier 30, more sendingamplification without increasing the total amplifier output power to avalue higher than it originally had, and moreover, in general withoutexceeding the load limit even though the transmitting line of thechannel originally operated at its load limit. From the standpoint ofload, then the energy represented by the curve 9 is, in general,equivalent to that of undistorted speech of some value less than g; sayit. We may then, by means of amplifier 80, introduce sendingamplification equal to g-]i. When this is done the received speech, orspeech at the east end of the channel, is raised to (Z where Thedifference in level between the curves a and 0 is then a measure of therelative magnitudes of speech and noise with the networks inserted. Itfollows then that the reduction in the effect of the interferenceobtained by inserting the networks is measured by c0 is the reduction inapparent loudness of the noise caused by the receiving network. Itdepends on the distribution of the interfering energy as well as on thecharacteristic of the network.

wa is the difference between the combined attenuations introduced by thenetworks and the decrease in equivalent load which results from passingundistorted speech through the sending network. This decrease dependsupon the distribution of the speech energy, the shape of the networkcharacteristic and the manner in which overloading occurs at the sendingend.

It is interesting to consider the case where the receiving network issuch as to give the noise at the receiver the distribution of speech;that is, to make Z2 a straight line. Then the speech energy at thesending end 9 will have the same shape as the interference 6. Such apair of networks will give an imto the-manner in which overloadingoccurs with the particular apparatus employed at thesending end. 7

In case the attenuation in the channel, between network 20 and network21, varies with the frequency we may treat the receiving end exactly asbefore. To render the overall ransmission of the channel distortionlesswe must, however make the distortion. of the sending network thecomplement of that of the receiving network plus the variable part ofthe attenuation frequency characteristic of the channel, between thenetworks. The distribution of sending power will then dif fer from {/2by the distortion of the channel between network 20 and network 21. Thevalue of the load equivalent it will then in general be different fromthat for a distortionlesschannel. Hence the optimum distortin networkwill depend on the attenuation requency characteristic of the channel aswell as the factors already discussed.

It will be understood that the showing of the attenuating networks inthe drawing is merely diagrammatic, and that any suitable networks maybe employed. I

The broad features disclosed herein may be embodied in forms widelydifferent from that specifically shown and described above, withoutdeparting from the spirit of the invention defined in the followingclaims.

hat is claimed is:

1. The method of reducing the disturbing effect upon signaling waves ofinterfering waves, which comprises changing the frequency distributionof the energy of only the signaling waves, while maintaining the totalenergy of said altered waves at least as low as that of said signalingwaves before said alteration, and subjecting the altered signaling wavesand the intefering waves to distortion of such character as to give saidaltered signaling waves their original form and to reduce the energylevel of the undistorted signals of which the interfering Waves are anequivalent as regards apparent volume, by an amount greater than thereduction caused by the two changes in the energy level of said signals.

2. The method of reducing the disturbing effect of interfering wavesupon signaling waves for transmitting speech, which coinprises changingthe frequency distribution of the energy of only the signaling waves,and subjecting the altered signaling waves and the interfering waves todistortion of such character as to compensate for the dis tortionsuffered by said signaling waves in the first mentioned change, and toreduce the energy levelof the undistorted speech of which theinterfering waves are an equivalent as regards apparent loudness,anamount greater than the reduction caused by the two changes of saidsignaling waves in the energy level of said signaling waves.

3. The method of transferring signaling waves through a signalingchannel traversing a reglon of interfering waves, which compriseschanging the frequency distribution of the energy of said signalingwaves in said channel before they enter said interference region andwhile maintaining the load imposed on said channel by said alteredsignaling waves at least as low as that imposed on said channel by saidsignaling waves before said change, subjecting the signaling waves andthe interfering waves, after they leave the interference region, todistortion complementary to the distortion occasioned by the firstmentioned change and of each character as to reduce the apparent volumeof said interfering waves in said channel by an amount greater than theresultant decrease of signal volume occasioned by the changes sufferedby said signal waves before entering and after leaving said interferenceregion. p

4. A signaling wave transmitting channel having a sending end, areceiving end, a portion between said ends traversing a region ofinterference, distorting means at said receiving end, means introducingcomplementary distortion at said sending end, and an amplifier at saidsending end and between the two distorting means, the characteristics ofsaid two distorting means being such that said receiving end distortingmeans reduces the apparent volume of received interference waves anamount greater than the difference between the combined apparent signalvolume decreases caused by said two distorting means and the apparentsignal volume gain increase which the introduction of said sending enddistorting means renders possible in said amplifier without increasedtendency to overload the channel.

5. A duplex radio telephone system comprising two terminal circuits andtwo unidirectional, oppositely directed signaling channels connected tosaid circuits in conjugate re.ation to each other, each of said channelshaving means for reducing the disturbing effect of radio interferencewaves upon the articulation of signals transmitted through that channeland each of said means comprising a non-uniformly attenuating network atthe outgoing end of one of said channels, a complementary network at theincoming end of said one channel, for predistorting the'signals in amanner opposite to that in which they are distorted by said attenuatingnetwork at said outgoing end of said one channel and means at theoutgoing end of said channel for amplifying the signals to compensatefor the attenuation produced therein by the networks at the incomingandoutgoing ends of said channel, said networks being designed with respectto the relative energy level of said signal and said interfering wavesso that a minimum amount of amplification by said amplifying means willbe required to secure a given standard of operation for said system.

6. A signaling system for transferring signals over a region ofinterference, said system comprising means for reducing the disturbingeffect of saidinterference upon the signals received over the system,while maintaining the load on the sending end of the system, withrespect to the load limit of said end, at least as low as would obtainin the absence of said means, said means comprising stationary means atthe sending end of the system for distorting the signals and stationarymeans at the receiving end of the system for restorin the form of thesignals and changing the frequency distribution of the interferingenergy arriving at the receiving end of the system.

7 A signaling system for transferring sig nals over a region ofinterference, said sys tem comprising a transmitting station, areceiving station, and means for reducing the disturbing effect of saidinterference upon the signals received over said system, said meanscomprising means at said transmitting station for changing the frequencydistribution of the energy of said signals, and means at said receivingstation for restoring the form of the signals and changing the energylevel of the interfering waves arriving at said receiving station withrespect to the change in the energy level of the signals transmittedover said system by an amount large in comparison with the resultantchange in energy level of the signals produced by the means for changingthe frequency energy distributions of the signals at the transmittingand receiving stations.

8. The method ofreducing the disturbing effect of interfering waves onsignals transmitted from a sending station to a receiving stationthrough an interference zone, which comprises changing the frequencydistribution of the energy of said signals at said transmitting stationand, at said receiving station, restoring the. form of the signals andchanging the energy level of the interfering waves by an amount large incomparison with the resultant change in the energy level of the signals9. The method of transmitting signaling waves through interfering waves,which comprises relatively changing the energy of only the signalingwaves at different frequencies, and subjecting the altered signalingwaves and the interfering waves to distortions of such character withrespect to said change in the energy of the signaling waves as to renderthe disturbing effect of the interfering waves and said change and saiddistortions less than the disturbing effect of the interfering waves inthe absence of said change and said distortions.

10. The method of reducing the disturbing effect of interfering wavesupon intelligence transmitting waves in an intelligence transmissionsystem, which comprises so distorting the intelligence transmittingwaves before they encounter the interfering waves as to cause therelation between the energy levels of the intelligence transmittingwaves at their different frequencies to substantially more closelyapproach the relation between the maximum permissible energy levels inthe system at those frequencies, and compensating for at least a portionof said distortion.

11. The method of reducing the disturbing effect of interfering wavesupon signaling waves in an intelligence transmission system, whichcomprises so distorting the signaling waves before they encounter theinterfering waves as to substantially increase the amount that thesignaling waves can be amplified without exceeding a given degree ofoverloading of the system, amplifying the altered signaling waves beforethey encounter the interfering waves, and compensating for at least aportion of the distortion after the signaling waves have encountered theinterfering waves.

12. The method of reducing the disturbing effect of interfering wavesupon telephone signaling waves in. a telephone transmission system,which comprises so distorting the telephone signaling waves before theyencounter the interfering waves as to substantially increase the amountthat the telephone signaling waves can be amplified without exceeding agiven degree of overloading of the system, after such change subjectingthe telephone signaling waves to the action of said interfering waves,and so changing the resulting waves with respect to their relativevalues of energy at different frequencies as to make their frequencydistribution of energy more nearly approach a given standard.

13. The method of reducing the disturbing effect of interfering wavesupon intelligence transmitting waves, which comprises rendering thefrequency distribution of the energy of the intelligence transmittingwaves much more nearly uniform before they encounter said interferingwaves, after such change subjecting the intelligence transmitting wavesto the action of said interfering waves, and so changing the resultingwaves with respect to their relative value of energy at differentfrequencies as to make their frequency distribution of energy morenearly approach a given standard.

14. The method of transferring signaling. waves through a signalingchannel traversing a region of interfering waves, which comprisesrendering the energy of the signaling waves substantially less variablewith frequency before they enter said interference region, restoring theshape of their energy frequency characteristic after they leave saidlnterference region, and attenuating said 1nterfering waves in saidchannel.

15. A wave transferring system having a passive network of lumpedimpedances at one end thereof for so distorting said waves as tosubstantially increase the amount that the signaling waves can beamplified without exceeding a given degree of overloading of the system,means at said end of said system for amplifying said distorted waves,and a passive network of lumped impedances at the other end thereofeffecting a complementary change in said waves. 7

16. The method of reducing disturbing effects of interfering waves onsignaling waves, which comprises subjecting the signaling Waves beforethey encounter the interfering waves to distortion of such character asto reduce by a given amountthe load equivalent of the signaling waveswith regard to the manner in which they tend to cause overloading in thesystem, and subjecting the interfering waves and the signaling wavesafter they encounter the interfering waves to distortions of suchcharacter as to compensate for the distortion suffered by the signalingwaves in said first mentioned distortion and to reduce the apparentenergy level of interfering waves as measured by their disturbing effectby an amount exceeding the com bined energy level decreases suffered bysaid signaling waves in said distortions before and after encounteringsaid interfering waves minus said reduction-in load eqnivalent.

In witness whereof, I hereunto subscribe my name this 26th day ofDecember A. D.,

RALPH v. L. HARTLEY.

